1. Field of the Invention
The invention described herein relates to the field of piezoelectric transducers. More specifically, the invention relates to ceramic piezoelectric transducers and especially to piezoelectric transducers employing lead zirconate titanate (PZT) ceramics.
2. Description of the Prior Art
Piezoelectric materials can be used both for (1) sensing sound and producing electrical signals representative of the sound and for (2) converting electrical signals into sound, whereby sound is projected or transmitted from the piezoelectric materials.
A particular type of ceramic material is here of special interest. This type of material has ordered voids with alternating planar ordered void regions and planar ordered solid regions. The solid regions between the voids are dense ceramic. One such ordered void ceramic material is an ordered void PZT ceramic material which is the subject of U.S. Pat. No. 4,683,161, incorporated herein by reference. This ordered void piezoelectric material is especially suitable for forming piezoelectric PZT high sensitivity acoustic sensors, usable as hydrophones in a high pressure underwater environment. The ordered void structure permits the PZT material to undergo a degree of compression in a direction at 90 degrees to the electrodes and thereby perform the piezoelectric sensing functions in a way that is significantly superior to what it would be without the voids. Nevertheless, the PZT ceramic having the ordered void structure does not perform very efficiently as an acoustic transmitter. This is so because the solid regions in between or under the voids are not subjected to the same electric field as the PZT material next to the voids when the material is poled or driven with an A. C. field. More specifically, the voids themselves are of low dielectric constant, and the solid regions of ceramic below and between the voids have a much higher dielectric constant. With an externally applied field, the low dielectric constant of the voids causes the electric fields in these voids to attain a relatively high value; and the electric field in the ceramic below and between the voids that are in series with these voids attains a very low value. The latter is then much lower than the field in the solid, void free ceramic columns next to the voids. The lack of uniform electric field distribution both impedes efficient poling of the PZT ceramic material and prevents uniform and low loss A. C. excitation. It would be desirable, therefore, to provide an ordered void PZT ceramic piezoelectric material that permits an applied electric field to more efficiently excite the solid regions in the PZT material that are in between or under the voids so that the piezoelectric material is more uniformly poled and can adequately serve as an acoustic transmitter.
There are composite ceramic materials disclosed in the prior art in which voids in the material are entirely filled with metal (e.g. solder). A representative patent disclosing such a composite material is U.S. Pat. No. 3,879,645 of Rutt et al which discloses monolithic ceramic capacitors that are formed by producing sintered ceramic bodies with voids. The voids are produced during sintering. The voids may have distinct pillars which remain after sintering. The voids are filled with a metal solder alloy to form conductive electrodes. The solder metal filling increases the compressive modulus of the ceramic voids, so that such a filled composite material exhibits decreased compressibility; and such decreased compressibility provides for a poor sensor.